Resonant cavity tuning device



June l', 1948. H. E. ToMPKlNS 2,442,671

RESONANT CAVITY TUNING DEVICE Filed Feb. 29, 1944 2 sheets-sheet 1 5m@ 1948- H.. ToMPKlNs RESONANT CVAVIIT!v TUNING DEVICE 2 Sheets-Sheet 2 Filed Feb. 29, 1944 Patented June 1, 1948 RESONAN T CAVITY TUNING DEVICE Howard E. Tompkins, Ridley Park, Pa., assigner, by mesne assignments, to Philco Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application February 29, 1944, Serial No. 524,472

9 Claims. (Cl. 178-44) This invention has to do most particularly with the art of adjustably tuning electrical resonant cavities and its primary object is to provide a manually operable device of simple and compact construction which is capable of varying the resonant frequency of such a cavity throughout a band of considerable width, and which, inter alia, can be employed eiectively to that end in certain high frequency radio equipment hereinafter described, with respect to which previously known tuning devices have been found inadequate.

Heretofore, adjustable tuning of high frequency resonant cavities has most often been accomplished by one or the other, alternatively, of two distinct types of devices. One such prior art device is operative to alter the eiiectiveness of the magnetic field within the cavity and thus vary the inductance, substantially to the exclusion of any eiiect upon the electric iield, while the other is operative as a variable condenser to alter the electric iield within the cavity, substantially to the exclusion of any eifect upon the magnetic field. As a rule, it was the prior art practice to equip the cavity with a tuning device of one or the other aforementioned types, although it is believed there may have been instances where both types of tuners were employed on a single unit of equipment.

Where conditions are favorable, the previously known tuning devices to which reference has been made are ordinarily capable of aifording all the tuning range likely to be needed; but on the other hand, where certain adverse structural conditions obtain, as illustrated by the high frequency velocity-modulated oscillator herein dealt with, it has been found inconvenient or impracticable to achieve enough tuning range with such of the prior art tuning devices as were readily capable of being installed.

The present invention is characterized in that it affords in a -singl-e small unit a device which, in response to manual operation, will Vary simultaneously and complementively both the magnetic iield and the electric i'leld of a resonant cavity with the result that a considerably wider band of frequency variation is achieved than could be realized under like conditions with a tuning device which affects mainly or only the magnetic iield. or which aifects mainly or only the electric eld, and which could, at the same time, actually be employed notwithstanding the unavoidable adverse structural conditions with which the invention primarily was intended to cope.

Although the need for the present invention is as to inhibit the eii'ective use of previously known tuners, it will be noted that the simplicity of the subject device is so marked that its use often can be'recommended even where necessity does not obtain.

While the present invention is especially useful for the tuning of resonant cavities'it will become apparent as the description proceeds that its utility is not limited in that respect.

Referring to the drawings:

Fig. 1 is a plan view of a simple resonant cavity provided with a tuner in conformity with the present invention;

Fig. 2 is a cross-sectional View taken along the line 2-2 of Fig. 1 and showing an end view of the tuner in the posture which effectslthe minimum resonant frequency;

Fig. 3 is identical with Fig. 2 except that the tuner is shown in its alternate extreme posture wherein it effects the maximum resonant frequency;

Fig. 4 is a fragmentary cross-section taken through the tuner and a portion of the cavity structure along the line 4-4 of Fig. 2;

Fig. 5 is an enlarged perspective view of the rotatable tuning element;

Fig. 6 illustrates an alternative tuner construction;

Fig. '7 is a plan view of a velocity-modulated oscillator including an embodiment of the invention;

Fig. 8 is a View partly in vertical section and partly in elevation of the structure of Fig. 7;

Fig. 9 is an enlarged detail in longitudinal jsection of the tuner element included in the structure of Figs. 7 and 8;

Fig. 10 is a cross-section taken along the line IU-IB of Fig. 9;

Fig. 11 is an endwise View of another form of tun-er in accordance with the invention;

Fig. 12 is a view similar to Fig. 11 but showing the tuner rotated degrees; and

Fig. 13 is a sectional View taken along the line |3-I3 of Fig. 11.

In Figs. 1 to 4 inclusive there is shown a very simple resonant cavity structure which is not intended to illustrate a commercial embodiment of the invention but rather to facilitate exposition and comprehension of the underlying principles involved. Deiiciencies of mechanical design should accordingly be ignored so far as Figs. 1-4 are concerned.

The metal cavity structure I is cylindrical and quite short axis-wise and encloses a cylindrical most evident where structural environmentissuch $5 chamber 2 which constitutes the cavity per se.

arrows 4 in Figs. 2 and 3, and by the dots 4 asindicated in Fig. 1. As willbe observed, the course of the magnetic field is perpendicular to the longitudinal axis of the cavity while the course ofV the electric eld is parallel tcjsaid axis and perpendicular to the magnetic field. The fact that the electric eld is most intense at the axis of the cavity and of gradually diminishing intensity toward the periphery is indica-ted by the varying density of the dots 4 in Fig. 1. For simplification eld displacements caused Iby the tuner have not been depicted.

Adjustable tuning of the cavity is effected by means of a U-shaped metal element 5, preferably non-magnetic, which is sh'own in perspective, greatly enlarged, in Fig. 5. Element 5 is disposed inside the cavity and adjacent the periphery thereof as indicated in Fig. 1, and is manually rotatable about its axis of rotation-which axis extends transversely of both the magnetic field and the electric field. Rotatably supporting element 5 is a spindle B- of dielectric material, such vas Bakelite, having a portion 6a of reduced diameter which is journaled in an opening in the cavity wall and provided at its outer end with a knob 1. Element 5 comprises two parallel diametrically opposed arm-like members 5a and 5b spaced symmetrically about the-,axis of rotation and interconnected by a cross-piece 5c which is integral with the arms.

Element 5' performs, simultaneously, two operations which contribute additively to variation ofl the resonant frequency of the cavity. More specifically, elementV 5 is operative in response to rotation to alter the effectiveness of the magnetic eld while at th'e same time complementively altering the electric eld. The result is comparable, on the one hand, to that which is achieved by adding inductance and capacity to an ordinarytuned L/C circuit or, conversely, reducing both the inductance and capacity of such a circuit.

As clearly depicted in Figs. 2 and 3, the cavity structure is bored at 8- and 9 to form arcuate concave surfaces which narrowly clear the complementary outer arcuate convex surfaces of members 5a and 5b; and, as will be apparent upon inspection of Fig. 2, two resultant seriesconnected capacities at i and Il, respectively, are interposed betweenvthe endwalls of the cavity when element assumes the posture in which it is shown in that figure,- Manifestly, the capacity thus added substantially increases the aggregate electrostatic capacity of the cavity, even though' element 5 is located largely outside the region of high electric field. The magnitude of this added capacity is accentuated by the fact that the peripheral areas of members 5a and 5b are relatively large and are disposed in their entirety in closely spacedA vis-a-vis proximity to the cooperating end wall surfaces. It will at once be evident that so far as the electric field of the cavity is concerned, the resonant frequency attains a minimum value when element 5 assumes the posture of Fig. 2. In that posture members 5a and 5b are to a maximum extent removed from the path of the magnetic field, as will be apparent from inspection of Fig. 2, and consequently said arms do not then operate to any substantial extent as short-circuited conductors in the magnetic field and accordingly do not cause any material displacement of that eld. Therefore, the magnetic field and, correspondingly, the inductance of the cavity, are most pronounced wh'en the electric eld is at maximum strength; and the resonant frequency is consequently at its minimum value-provided it be assumed that only one tuner is employed, as shown..

When element 5 is rotated upon its axis to the posture in which it is shown in Fig. 3, members 5a and 5b are withdrawn to the greatest possible degree from vis--vis proximity to the respective end wallsl of the cavity and it will be apparent that the tuning -capacity produced by those arms in conjunction with the complementary end wall surfaces is then minimized, The effect of such capacity reduction is, of course, to increase the resonant frequency. While the capacity is reduced to a minimum value, the inductance ofthe cavity is also minimized because the full inter-- position of members 5a and 5b lying athwart the magnetic eld, as shown in Fig. 3, causes a maximum displacement of th'e magnetic eld. This is due to the fact that said members are then so related to the magnetic ileld that they behave as short-circuited conductors therein. The displacement of the magnetic eld and' concomitant lessening of inductance also contributes to an increase of resonant frequency. Hence, the resonant frequency attains a maximum value when element 5. assumes the posture in wh'ich it is shown in Fig, 3. Withelement 5l postured at any angle of rotation between the two extremes of Figs. 2 and 3, respectively, the resonant frequency will attain, at least approximately, a proportionate value. Obviously, it is unnecessary to provide for rotation of element 5 through more than degrees since further rotation in the same direction after completing a 90 degree turn would have the same effect as reverse rotation through the original quadrant. The respective rotational postures of the tuner exhibited in Figs. 2 and 3 are, for convenience, referred to hereinafter as extreme postures.

On its face, it would seem that the locus of cross-piece 5c along the axis of rotation of element 5 should be immaterial; but in working at hyper-frequencies it has been found that such is not th'e case. If cross-piece 5c is relocated more remotely from the center 0f the electric field, which is to say nearer knob 1, it will be discovered that an apparent reduction of capacity results. The observed phenomenon is not in reality due to capacity reduction but is believed to be due to current lag resulting from the inductance of element 5, which inductance is less effective when cross-piece 5c is situated between points of higher potential (as per Fig. 4) than when said cross-piece is moved to a position more remote from the center of the electric field where the A, C. potential at its terminals is not so high. Accordingly, it is a feature of this invention to locate cross-piece 5c between the'high potential ends of arms 5a and 5b in preference'to locating said cross-piece between points of lower potential.

It should be somewhat obvious from the foregoing exposition that increased tuning range could be achieved by extending the tuner farther toward the center of the electric field when that lis physically possible, but where high power levels are involved it may not be desirable to do so because of possible spark-overs. However, it is not intended to exclude from the purview of this invention tuners which extend into the high voltage area.

To achieve maximum effectiveness most conveniently it generally is preferable that each arm 5a and 5b subtend an angle of approximately 90 degrees about the axis of rotation, as viewed in Fig. 2, and that the depth of the cavity as viewed in that gure be approximately equal to the chord of the arc subtended 'by each arm. This facilitates attainment of maximum tuning range with the most simple and compact mechanical design, but it is not an indispensable factor and is not to be construed as a limitation upon the scope of the invention.

An alternative mode of supporting the tuning element 5 is illustrated in Fig. 6 wherein said element is shown connected at its mid-point to one end of a metal spindle l2 which is journaled at kI3 in the cavity Wall and provided with a knob |4 at its outer end. 'Ihe point of attachment of spindle l2 to element 5 is at the same instantaneous potential as the point where the spindle contacts the cavity wall. Hence, it is not necessary to employ a spindle of dielectric material, but it is essential that spindle l2 be connected only to the mid-point of element 5 and not to any point thereon having an instaneous potential diering from that of the cavity wall at the area of contact with the spindle.

In Figs. 7 and 8 there is shown an adaptation of the invention to a velocity-modulated oscillator tube; and Figs. 9 and 10 are enlargeddetail views of the tuner. This is a special case wherein a tuner according to the present invention was found to afford an adequate tuning band wherea's, on account of structural limitations, prior art tuners capable of being installed did not prove sufficient.

The oscillator of Figs. 7 and 8 comprises a vacuum tube I5, including a glass envelope l5 having a base I1 and enclosing a number of elements, as follows: a cathode heating filament I8, cathode I9, accelerating grid 28, resonant cavity grids 2| and 22 and a repeller 23.

Grids 2| and 22 are supported on metal diaphragms 24 and 25 respectively, corrugated as shown and extending through the glass envelope and sealed thereto at the junctures. The external portions of diaphragms 24 and 25 constitute thin metal iiange-like fins completely encircling the envelope and projecting radially therefrom. Said fins afford means of attachment from an external structure 26 which, conjointly with diaphragms 24 and 25, forms a resonant cavity for determining the oscillation frequency.

Structure 26 is annular, as viewed in Fig. 7, and comprises two complementary semi-annular members 21 and 23 of U-shaped cross-section, two semi-annular lower clamping plates `29 and 30, and an annular upper clamping plate 3|, together with a number of clamping screws 32. The function of the clamping plates and screws is to secure cain'ty members 21 and 28 to the external peripheral edges of diaphragms 24 and 25. The area defining the cross-section of the cavity per se is indicated in Fig. 8 by reference numeral v3?, and this includes the space between diaphragms 24, 25, both inside and outside envelope IB and the space between grids 2| and 22, as well as the rectangular space defined by members V21 and 28 beyond the limits of said dlaphragms.

A tuner 34 such as previously described is inserted into the cavity through the peripheral wall of member 21 and is held in position by means of a threaded bushing 35. Details of tuner 34 are shown enlarged in Figs. 9 and 10, and should require no further description in view of the preceding descriptive matter respecting Figs. 1 to 6.

It has been found desirable at times to provide, in addition to tuner 34, a second tuner of the same character but of smaller dimensions and designed to cover a much smaller band. The second tuner is in the nature of a trimmer and facilitates exactness of tuning lbesides adding something to the width of the tuning band. Such a second tuner is indicated at 35 in Fig. 7. It will, of course, be understood that as many of the subject tuners as desired may be employed in a single cavity.

For the purposes of the Ipresent invention it is unnecessary to expatiate upon the Inode of operation of the oscillator, it being thought suincient to mention that it is one which is sometimes denominated a reflex Klystron.

To be of maximum effectiveness, it is necessary that the axis of rotation of the tuner extend perpendicularly to b-oth the magnetic and electric lines of force within the cavity, 'but analysis will show that these are not exacting limitations since the tuner will still be highly effective even though its axis of rotation is only approximately so oriented.

While nearly always it is preferred to employ a tuner of one or the other species hereinbefore described, there may be occasion to resort to still another alternative such as that illustrated in Figs. 11-13 which comprises only one conductive member 31 corresponding to one of the arm-like members 5a and 5b of Fig. 5. Here the parallel walls of the resonant cavity are identified respectively by reference numerals 38 and 39. The arcuately shaped conductive member 31 is attached by means of a screw 4E] to a spindle 4| of dielectric material such as Bakelite. The end of spindle 4| is slotted radially to receive a exible ribbon conductor 42 which is connected at one end to member 31 and at the other end to the surface of the lower cavity wall. The ilexible conductor permits free rotation of the tuner through a quarter turn whereby to enable movement of member 31 from the posture shown in Fig. 11 to that of Fig. 12. Member 31 functions both as a movable condenser plate and, when in the posture of Fig. 12 and intermediate postures, as a short-circuited conductor athwart the magnetic field. Manifestly, the principle of operation is identical with that of the tuners previously described herein; but it has at least one point of advantage thereover in that size-for-size and equal plate separation the magnitude of variable tuning capacity is doubled.

I claim:

1. In combination, structure defining a pair of spaced, opposed relatively fixed conductive surfaces, means for engendering a high-frequency electric eld between said surfaces together with a co-ordinate high-frequency magnetic eld directed transversely to said electric field within the space between said surfaces, and a rotatable tuner disposed within said space and having its axis of rotation extending transversely to both the aforementioned fields, said tuner including a conductive member laterally displaced from said axis; of` rotationV and revoluble thereabout between a rst extreme posture and a secondK ex-V treme posture, said member beingv so formed and'. proportioned that when it assumes theiirst ofsaidextreme postures it isrelativelyineffective uponv said magnetic field, said member` having a surface of relativelyl large area which in` said-first. posture is in maximum vis--vis relation to one ofV said fixed surfaces to form a tuning capacity therewith, said member being so formed andl proportioned that in its second extremeposture it presents a relatively large area athwart said magnetic field and has a minimum ofV area in vis-a-vis relation to the lastmentioned1 one of said fixed surfaces, said member insaidsecond posture being reactively related to said magnetic field to aY high degree and operative as a short-circuited secondary loop conductor in said field, said member in said first posture'being included in a shunt path between said fixed surfaces, which shunt path includes saidtuning capacity in series with said member and in series with a conductor substantially bridging the gap between said member andj the onev of said fixed surfaces other than that with which said member forms said tuning capacity.

2. Incombination, structuredefining a pair of spaced, opposed conductive surfaces, means for engendering a high-frequencyr electric field' -between saidy surfaces together with a co-ordinate high-frequency magnetic field directed transversely tosaid electric field within the space between said surfaces, and a rotatable tuner disposed within said space and having its axis of rotation extending transversely to both the aforementioned fields, said tuner including a pair of conductive members spaced apart and spaced laterally from said axis and at different sides thereof, said members being proportioned and oriented to define, conjointly, a substantially clear passageway for said magnetic eld when said tuner is adjusted to a predetermined first extreme posture, said memberseach being proportioned and' oriented to form with one of saidl surfaces individually a supplemental capacity of' substantial magnitude when said tuner is ad'- justed according to said rst posture, said memberslv being electrically interconnected to place said supplemental capacities in series, said tuner being rotatably adjustable to a predetermined second extreme posture wherein said members lie substantially athwart said magnetic eld and wherein the aforementioned capacities are minimized, said members being reactively related to said magnetic eld to a high degree and operative as secondary loop conductors to substantially affect said field'when said tuner is adjusted to the aforesaid second posture.

3. The combination according to claim 2 characterized in that said members are joined through a conductor which is connected to said members only at or near the respective ends thereof which are disposed most nearly the. region of maximum electric field intensity.

4. In combination, structure dening a pair of spaced, opposed conductive surfaces, means for engendering a high-frequency electric field between said surfaces toegether with a co-ordinate high-frequency magnetic eld directed transversely to said electric field within the space between said surfaces, and a rotatable tuner disposed within said spa-ce and having its axis of rotation extending transversely to both the aforementioned elds, said tuner including a pair of conductive members spaced apart and spacedlaterally, from. said axis and at opposite sides t thereof, said memberseach having-an outer arcuatev convex surface complementary to one of saidcon cave-surfaces and concentric therewith about saidV axis of rotation, said members being proportionedand oriented to define, conjointly, a substantially clear passageway for` said magnetic field when said tuner is adjusted to a. predeterminedrst ex treme posture, saidmembers. each having-itssaid convex surface in maximum vis-a-vis relation to onefofsaid concave surfaces, individually, lto formtherewith a supplementary capacity ofsubstantial magnitude when said tuneris adjusted1according to said first posture, said members being electrically -interconnectedto place said supplemental-capacities in series, said tunerbeing rotatably adjustable to a predetermined second extreme posture wherein said members lie substantially athwart said magnetic field and wherein said members. are substantially removed from their aforementioned vis--vis relation to said concave surfaces to minimize'said supplemental capacities, said members being reactively; related to saidmagnetic :heldI to a high degree and operative as secondary loop conductors to substantially affect saidield when said tuner is adjusted to said second posture.

5. In combination, structure defining a, pair-of spaced, opposed conductive, relatively fixed surfaces, means for engendering a,- high-frequency electricfield between said surfaces together with a co-ordinate high-frequency magnetic field di'- rectedv transversely to said electric field within the space between said surfaces, and a rotatable tuner-disposed Within said space and having its axis of rotation extending transversely to both the aforementioned fields, said tunerincluding; a U-shaped conductive element comprising a pair of parallel arms interconnected by a cross-piece, said arms being spaced apart laterally and dis,- posed parallel to said axis of rotation and. oriented symmetrically thereabout, said tuner having a rst extreme posture of rotation and a second; extreme postureof rotation and being continuously rotatable through all intermediate postures, said arms mutually definingl a substantially cleary intervening passageway for said magnetic field and being substantially ineffective upon said magnetic iieldi when said tuner conforms to said first pos-- ture, said arms being disposed substantially athwart said magnetic field and operative in substantial degree as short-circuited loops in said magnetic field when said tuner conforms to said secondv posture, said arms each presenting a considerable surface area in vis--vis juxtaposition to one of said fixed surfaces individually and forming conjointly therewith a tuning capacity of substantial magnitude when said tuner is conformed to said first posture, said tuningcapacity being gradually reduced and the influence` of said arms on said magnetic field being gradually increased as said tuner is rotated from said first posture toward said second posture.

6. In combination, structure dening a pair of spaced', opposed, conductive, relatively fixed surfaces, means for engendering a high-frequency electric field between said surfaces together with a co-ordinate high-frequency magnetic field directed transversely to said electric field within the space between said surfaces, and a rotatable tuner disposed within said space and having its axis of rotation extending transversely to both the aforementioned fields, said tuner including a conductive member spaced laterally from said axis of rotation andrevoluble thereabout betweenA two extreme postures of adjustment, said member being so formed that in one extreme posture it presents a surface of considerable area in vis-avis juxtaposition to one of said fixed surfaces to form therewith a tuning condenser of substantial capacity and while in the same posture effects minimum displacement of said magnetic eld, said member, when revolved about said axis of rotation to its alternate extreme posture, being largely or wholly retracted from said fixed surface, thus minimizing the capacity of said tuning condenser, said member in said alternate posture being disposed in said magnetic field and operative therein as a short-circuited loop substantially affecting said magnetic field, and a conductor directly connecting said member to the other of said fixed surfaces.

7. The combination according tc claim 6 wherein the last-mentioned conductor is flexible and xedly connected to said member and said other xed surface and is adapted to permit rotation of the tuner while avoiding contact disturbances.

8. In combination, structure defining a pair of spaced, opposed, conductive, relatively fixed surfaces, means for engendering a high-frequency electric eld between said surfaces together with a co-ordinate high-frequency magnetic field directed transversely to said electric eld within the space between said surfaces, and a rotatable tuner disposed within said space and having its axis of rotation extending transversely to both the aforementioned fields, said tuner including a conductive member spaced laterally from said axis of rotation and revoluble thereabout between two extreme postures of adjustment, said member being so formed that in one extreme posture it presents a surface of considerable area in visa-vis juxtaposition to one of said fixed surfaces to form therewith a tuning condenser of substantial capacity and while in the same posturey effects minimum displacement of said magnetic eld, said member, when revolved about said axis of rotation to its alternate extreme posture, being largely or wholly retracted from said fixed surface, thus minimizing the capacity of said timing condenser, said member in said alternate posture being disposed in said magnetic field and operative therein as a short-circuited loop substantially aiecting said magnetic eld, and means electrically coupling said member to the other of said fixed surfaces, whereby to include said tuning condenser in a shunt path between said fixed surfaces.

9. In combination, a cavity resonator having high frequency electric and magnetic fields there in,'and a rotatable substantially U-shaped openended conductive tuning member disposed in said cavity resonator, the ends of said member being free of any circuit connection thereto, said member having its arms spaced symmetrically about its axis of rotation and being revolvable about said axis between two extreme postures of adjustment, said axis of rotation extending transversely to said magnetic eld, said member in one extreme posture effecting minimum displacement of said magnetic field, and when revolved about said axis of rotation to its alternative extreme posture, effecting maximum displacement of said magnetic field.

HOWARD E. TOMPKINS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 476,781 Broadnax June 14, 1892 996,408 Hamby June 27, 1911 2,280,824 Hansen et al Apr. 28, 1942 

